Acquiring the Bloody Baron's Gwent card

I know this has been covered quite a lot already, but I haven't been able to find a definitive answer to solve the issue for me.

Like many people, I've finished the Bloody Baron's quest line before getting his Gwent card. [SPOILERS AHEAD]. I messed it up and failed his final quest, so he's just sort of disappeared into Crookback Bog. So, obviously, he's not around to play any more.

Apparently his card should be waiting in his room, in a box on his desk. But I've been everywhere inside that building and can't find it anywhere. The 'Velen's Players' quest just says the next person on my 'To Fleece' list is the Baron. I've also seen that I can advance the quest via a noticeboard, but that seems quite unspecific.

So I'm a bit stuck. I really want to get his card and push on with his quest. How can I do that?

Thanks.